This invention pertains to microlithography in which a pattern, defined on a mask or reticle, is transferred to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of semiconductor integrated circuits and displays. More particularly, the invention pertains to suppression of aberrations arising from increased beam deflection used for correcting errors in stage-position control.
In recent years, as semiconductor integrated circuits increasingly have become miniaturized, the resolution limits of optical microlithography (i.e., microlithography performed using ultraviolet light as an energy beam) increasingly have become apparent. As a result, considerable development effort is being expended to develop microlithography apparatus and methods that employ an alternative type of energy beam offering prospects of better resolution than optical microlithography. One candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam. The charged particle beam passes through a charged-particle-beam (CPB)-optical system from a source (e.g., electron gun) through a reticle to a substrate (e.g., semiconductor wafer). Typically, the CPB-optical system includes an illumination-optical system that directs the beam from the source to the reticle, and a projection-optical system that directs the beam from the reticle to the wafer.
It currently is impossible to provide a CPB-optical system having an optical field large enough to expose an entire die pattern at one instant while achieving adequate control and minimization of aberrations. Hence, the pattern as defined on the reticle is typically divided into multiple small regions (subfields). Such a reticle is termed a xe2x80x9cdividedxe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle, in which each subfield defines a respective portion of the overall pattern. The subfields normally are illuminated one at a time and thus sequentially xe2x80x9ctransferredxe2x80x9d onto the wafer. During transfer of the subfield images to the wafer, the wafer is mounted on and appropriately moved by a wafer stage to accurately place the subfield images on the wafer. As projected on the wafer, the respective images of the illuminated subfields desirably are arranged so as to be connected (xe2x80x9cstitchedxe2x80x9d) together in the proper order and arrangement so as to form the entire pattern on the wafer after completing exposure of all the subfields of the reticle. General aspects of this xe2x80x9cdivided-reticle pattern-transferxe2x80x9d technology are described, for example, in U.S. Pat. No. 5,260,151 and in Japanese Kxc3x4kai patent document no. Hei 8-186070.
In divided-reticle pattern transfer, to ensure accurate transfer of the pattern portion defined by a reticle subfield to the wafer, the image of the subfield must fall accurately within the respective target region on the wafer. Conventionally, the wafer is mounted on a wafer stage, and wafer-stage position is measured using a laser interferometer that can measure stage position very accurately. In actual practice, however, errors occur in the actual wafer-stage position. For example, a positional error can be 1 xcexcm in one or both of the X- and Y-directions. Conventionally, such errors are conventionally corrected using a deflector configured to impart a compensating deflection to the beam. This operation normally is repeated for each transfer of a subfield.
Unfortunately, the conventional remedies summarized above result in the positions on the wafer of the projected subfield images being shifted from their respective calibrated positions. Also, use of a deflector results in the beam being consistently laterally shifted from the ideal central coordinates of the projection-optical system, and an overall increase in aberration due to deflecting the beam.
In view of the shortcomings of conventional practice as summarized above, an object of the invention is to provide charged-particle-beam (CPB) projection-exposure apparatus and methods that adequately suppress aberrations arising from shifts of beam position from an xe2x80x9cidealxe2x80x9d (or xe2x80x9ctargetxe2x80x9d) deflection position (at which a subfield image would be accurately transferred by the projection-optical system) due to errors in stage-position control.
To such end, and according to a first aspect of the invention, CPB microlithography apparatus are provided. In a representative embodiment of such an apparatus, an illumination-optical system is situated and configured to illuminate a charged-particle illumination beam from a source onto a reticle defining a pattern to be transferred onto a sensitive substrate. The reticle is divided into multiple subfields each defining a respective portion of the pattern and each being individually illuminated by the illumination-optical system for transfer of the respective pattern portion (the illumination beam passing through the illuminated subfield forms a patterned beam propagating downstream of the reticle). The apparatus also comprises a reticle stage on which the reticle is movably mounted to allow the illumination-optical system to illuminate a region of the reticle with the illumination beam. The position of the reticle stage is detected using a reticle-stage position detector. The apparatus also comprises a projection-optical system situated and configured to project and focus the patterned beam onto a sensitive substrate. The projection-optical system comprises an image-positioning deflector. The substrate is movably mounted on a substrate stage to allow the patterned beam, passing through the projection-optical system, to form an image of the illuminated subfield at a respective location on the sensitive substrate. The position of the substrate stage is detected using a substrate-stage position detector. The apparatus also comprises a controller connected to the illumination-optical system, the projection-optical system, the reticle stage, the reticle-stage position detector, the substrate stage, and the substrate-stage position detector.
The controller is configured to perform several functions, as follows: (1) controllably operate each of the projection-optical system, the reticle stage, the reticle-stage position detector, the substrate stage, and the substrate-stage position detector, so as to transfer the pattern subfield-by-subfield in a sequential manner from the reticle to the substrate; (2) controllably energize the image-positioning deflector so as to arrange the images of the subfields contiguously on the substrate; (3) detect errors in positioning of one or both the reticle stage and substrate stage for exposing an image of a subfield on the substrate; (4) correct the stage-positioning errors for subfields in a range of subfields by appropriately energizing the image-positioning deflector; (5) in a memory, store data concerning the stage-positioning errors detected in the range; (6) calculate an error statistic concerning stage-positioning errors detected regarding the subfields in the range, so as to yield data concerning an error trend in the range; and (v7) during exposure of a subsequent range of subfields, recalling the data from the memory and utilizing the recalled data and the data concerning the error trend in a previous range to control positioning of the stages during exposure of the subfields in the subsequent range so as to minimize average errors in stage positioning in the subsequent range. The data stored in the memory can include data on image rotation detected for the subfields in the range. The statistic can be, for example, a sum of squares of effective errors detected for the subfields in the range.
The controller desirably directs operation of the image-positioning deflector so as to achieve, in each range, a deflection of the patterned beam sufficient to reduce deflection aberrations compared to deflection aberrations obtained during exposure of subfields in a previous range.
Also desirably, the subfields on the reticle are arranged into a plurality of stripes each comprising multiple rows of subfields, wherein each stripe constitutes a range.
According to another aspect of the invention, methods are provided for performing CPB microlithography in which a charged-particle illumination beam sequentially irradiates subfields of a segmented pattern-defining reticle, and a patterned beam, formed from the illumination beam passing through an illuminated subfield, is directed to a corresponding location on a sensitive substrate. Specifically, the methods achieve control of deflection-induced aberrations of the patterned beam. In a representative embodiment of such a method, the reticle is mounted on a reticle stage that moves the reticle as required to allow the illumination beam to illuminate a desired subfield of the reticle. Similarly, the substrate is mounted on a substrate stage that moves the substrate as required to allow the patterned beam to form an image of the illuminated subfield at a desired location on the substrate. Respective positions of the reticle stage and substrate stage are detected. An image-positioning deflector is provided that is operable to deflect the patterned beam as required to form the image of the illuminated subfield at a desired location on the substrate. While transferring the pattern from the reticle to the substrate, errors in positioning of one or both the reticle stage and substrate stage for exposing an image of a subfield on the substrate are detected. The stage-positioning errors are corrected for subfields in a range of subfields by appropriately energizing the image-positioning deflector. Data concerning the stage-positioning errors detected in the range are stored. An error statistic is calculated concerning stage-positioning errors detected regarding the subfields in the range, so as to yield data concerning an error trend in the range. During exposure of a subsequent range of subfields, the data are recalled from storage and utilized (along with the data concerning the error trend in a previous range) to control positioning of the stages during exposure of the subfields in the subsequent range so as to minimize average errors in stage positioning in the subsequent range.
Hence, the beam deflections required to adjust subfield-image positions sufficiently to correct for errors in stage-position control are stored for each subfield in a range (e.g., a stripe). The beam-correction deflections are statistically processed and any trends (e.g., stage-positional shifts in a specific direction) are analyzed. When exposing the next specific range (e.g., the next stripe), the position of at least one stage (e.g., the substrate stage) is adjusted in anticipation of the calculated trend. For example, by providing a positional offset in a specific direction, the shift from the calibrated position for the respective stage can be minimized, thereby reducing deflection-induced aberrations.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.